Some electronic cameras in the prior art use an electronic image display, such as a liquid crystal display (LCD), for image review both before and after capture. In these cameras, the LCD display is powered on prior to image capture, since the LCD is used to review the image prior to capture. For example, in the electronic camera described in U.S. Pat. No. 4,456,931, initial depression of the shutter release causes a motion image to be displayed on a monitor on the camera. Further depression of the shutter release causes a still image to be captured from the motion sequence, stored in a buffer memory, and displayed on the monitor. Complete depression of the shutter release causes the transfer of the still image in the buffer memory to a separate output memory system. In prior art cameras of this type, considerable power is dissipated in sustained operation of the image display for image review prior to capture.
Some digital cameras rely on an optical viewfinder for composing images and an image display, such as an LCD display, for viewing captured still images. For instance, in Ser. No. 08/803,342, entitled “Electronic Camera with Quick Review of Last Captured Image”, filed Feb. 20, 1997 in the name of the same assignee as the present application, an electronic camera has a working memory for storing a working image corresponding to the last captured image and a non-volatile memory, such as internal flash memory or a removable memory card. When an image is captured, the image data is stored in the working memory, and the image data is also immediately processed for storage in the non-volatile memory. The working image is displayed on a display screen in a quick review mode of operation in response to user actuation of a quick review switch. The display screen is thus kept in an inactive state until the quick review is requested, and returns to an inactive state after a predetermined time period or when the quick review switch is again activated. In this manner, energy to drive the display screen may be conserved.
In the Kodak DC25 camera, which includes an optical viewfinder and a color LCD display, a still image is read from its sensor and temporarily stored in a buffer memory. The image is then read from the buffer, processed, and stored on a removable memory card, which altogether typically takes 5 to 10 seconds. Since the color LCD requires a backlight in order to provide a bright image, it draws a substantial amount of power when it is turned on. To allow reduced battery drain, the DC25 camera includes a display switch so the user can manually turn the display on and off. In order to immediately view images captured by the DC25 camera, the user must first push the display switch to turn on the display before capturing an image. Therefore, the LCD display is powered even during the time the user is looking through the optical viewfinder to compose an image. Once the user presses a shutter button and takes an image, a low resolution version of the image from the buffer memory is immediately displayed on the LCD display. At the same time a high resolution version is being processed and stored in the removable memory card.
In order to reduce memory space, the DC25 camera includes an erase button that can be used to delete one or more images from the removable memory. If the displayed image is unacceptable, the user typically wants to immediately erase the unacceptable image and take a new picture of the same scene. With the DC25 camera, however, the user must wait until the high resolution processing is completed, which takes 5 to 10 seconds. Therefore, the user is not able to immediately take a new image, but must instead wait until the processing of the undesired image is completed. As a result, the camera wastes battery power by creating and storing an image file that will be immediately deleted. Moreover, the picture opportunity may be lost due to the interim wait.
In the type of camera that relies on an optical viewfinder for image composition, what is needed is an LCD based camera design that enables users to quickly view and (when desired) immediately erase newly taken images, while minimizing the camera battery drain by powering the LCD only when necessary to review an image.